Infectious recombinant paramyxovirus from antigenomic cDNA

ABSTRACT

The present invention relates, in general, to a methodology or the generation of nonsegmented negative-strand RNA viruses (Pringle, 1991) from cloned deoxyribonucleic acid (cDNA). Such rescued viruses are suitable for use as vaccines, or alternatively, as plasmids in somatic gene therapy applications. The invention also relates to cDNA molecules suitable as tools in this methodology and to helper cell lines allowing the direct rescue of such viruses. Measles virus (MV) is used as a model for other representatives of the Mononegavirales, in particular the family Paramyxoviridae.

This application is a continuation application of U.S. Ser. No.09/011,425, filed Sep. 15, 1998, now U.S. Pat. No. 7,402,429 which is a371 National Phase application of PCT/EP96/03544, filed Aug. 9, 1996,which claims priority from European application No. EP 9511 2559.0,filed Aug. 9, 1995.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates, in general, to a methodology for thegeneration of nonsegmented negative-strand RNA viruses (Pringle, 1991)from cloned deoxyribonucleic acid (cDNA). Such rescued viruses aresuitable for use as vaccines, or alternatively, as vectors in somaticgene therapy applications. The invention also relates to cDNA moleculessuitable as tools in this methodology and to helper cell lines allowingthe direct rescue of such viruses. Measles virus (MV) is used as a modelfor other representatives of the Mononegavirales, in particular thefamily Paramyxoviridae.

The invention provides the technology for construction of recombinantvaccine strains, in particular MV vaccine strains containing codingregions for the expression of epitopes or entire protein from otherviruses, bacteria, or parasites. It also demonstrates that chimeric MVstrains containing heterologous envelope proteins can be constructedsuitable for targeting cells not containing an MV receptor. Thus, inprinciple, plasmids based on the genome of MV, packaged in envelopescontaining proteins for targeting special cell types can be constructed,encoding gene products either lacking in genetically defectiveindividuals or toxic for targeted malignant cells.

By straightforward replacement of the MV-specific helper cell linesdescribed in this invention by cell lines expressing the cognateproteins encoded by other representatives of the Mononegavirales to berescued, any other member of this viral order replicating in vertebratecells can be used for the purpose of live vaccines or of vectors forgene therapy instead of MV.

2. Background Information

Measles Virus

MV is a member of the family Paramyxoviridae. Its genetic information isencoded on a single RNA strand of negative polarity, comprising 15894nucleotides. The genome is sequentially transcribed from the 3′ terminusto yield, in addition to a leader RNA, 6 major capped and polyadenylatedmessenger ribonucleic acid (RNA) species, each of which encodes onemajor protein. The genome map is shown in FIG. 1, indicating the genesspecifying as the principal products N (nucleocapsid protein), P(phosphoprotein), M (matrix protein), F (fusion protein), H(hemagglutinin) and L (large protein=polymerase). Several additional RNAand protein species, in part mentioned in the Table of FIG. 1 complicatethis simple picture, but they are not relevant here.

MV is a major cause of acute febrile illness in infants and youngchildren. According to estimates of the World Health Organisation (WHO),one million young children die every year from measles. This high tollarises primarily in developing countries, but in recent years alsoindustrialised countries such as the USA have been affected again bymeasles epidemics, primarily due to incomplete adherence to immunisationprograms (Clements and Cutts, 1995). At present, several live attenuatedMV vaccine strains are in use (including the Schwarz, Moraten andEdmonston-Zagreb strains), almost all derived from the originalEdmonston strain (Enders and Peebles, 1954) by multiple passage in nonhuman cells (Enders, 1962). For a recent discussion of MV vaccinologyincluding future trends see Norrby (1995). Measles vaccine is usuallyadministered at 15 months of age or, in developing countries, already at6 months, and has proved to be highly effective, usually providinglife-long immunity against MV reinfection eliciting morbidity. To date,the genetic alterations responsible for attenuation of these vaccinestrains remain unknown. The proven safety of measles vaccine, combinedwith its high and long-lasting efficiency, predestines it as an idealplasmid for the expression of heterologous genes. Such a vaccine mayprove as efficient in eliciting long-lasting immune protection againstother pathogenic agents as against the vector virus itself. Anotherpossible candidate as vaccination vector is Mumps virus, a distantrelative of MV, which is also highly efficaceous and safe as attenuatedlive vaccine.

Rescue of RNA Virus from Cloned DNA

The study of the replication cycle of a number of RNA viruses has beengreatly facilitated by the availability of DNA clones from whichinfectious virus can be rescued, thus allowing the application ofreverse genetics. Initially, the bacteriophage Qβ (Taniguchi et al.,1978) and polio virus (Racaniello and Baltimore, 1981), and subsequentlySindbis virus (Rice et al., 1987) were expressed from cloned cDNA. Todate, a large variety of positive-strand RNA viruses, primarilyinfecting vertebrates and plants, can be rescued from cloned DNA (for arecent review see Boyer and Haenni, 1994). In addition, proviral DNA ofretroviruses is infectious. However, attempts to obtain infectious virusfrom cDNA clones of negative-strand RNA viruses have met with greatdifficulties. This is due to two properties of these viruses: (i)neither genomic nor antigenomic RNAs are infectious, because they do notserve as mRNAs; and (ii) both transcription and replication requireribonucleocapsids, i.e., rod-like nucleoprotein complexes (RNPs),containing the genomic RNA and several proteins with structural and/orenzymatic function.

Rescue from cloned DNA has been achieved several years ago in the caseof influenza virus, a negative-strand RNA virus containing eight genomesegments. Their RNPs which are small in size and loosely structured asrevealed by the susceptibility of their RNA component to RNase, can beassembled in vitro from RNA and the required viral proteins, N and thepolymerase components. Initially, an artificial RNA has been usedcarrying as a reporter the chloramphenicol acetyltransferase (CAT)coding sequence embedded in the noncoding terminal segments of aninfluenza virus genome subunit (Luytjes et al., 1989). Later, singleauthentic or altered genome subunit RNAs transcribed in vitro fromcloned DNA were used (Enami and Palese, 1991). The assembled RNPsreplicated and transcribed upon transfection into influenza-infectedcells, as monitored by CAT production and by rescue of a reassertedinfluenza virus, respectively. Purification of virus containing theintroduced subunit from the vast excess of non-reassorted virus in somecases can be accomplished by selection, for example, using a specificneutralising antibody directed against the protein encoded by thecognate subunit of the helper virus.

In contrast, for the viruses with a nonsegmented negative-strand RNAgenome, grouped together in the order Mononegavirales (Pringle, 1991)the much more tightly structured and longer RNPs, containing in additionto the N protein the assembly and polymerase cofactor phosphoprotein (P)and the viral RNA polymerase (large protein, L) have been refractory tofunctional reassociation in vitro. Therefore, many laboratoriesapproached the rescue of representatives of the Mononegavirales startingout with subgenomic RNAs containing only essential sections of the viralgenomes, using viruses to provide the helper proteins required tointracellularly encapsidate and replicate these mini-replicons. First,naturally arising subgenomic RNAs, competing with the viral replicationand thus known as defective interfering particle (DI) RNAs (Re, 1991)were used, being substituted later by artificial DI RNAs containingreporter genes, transcribed from appropriately constructed plasmids.These mini-replicons, first devised by the group of M. Krystal (Park etal., 1991) according to the replicon used for the initial influenzarescue model (Luytjes et al., 1989), carry a CAT coding sequenceinserted into viral noncoding terminal regions of Sendai virus (SeV) andhave been used successfully also for respiratory syncytial virus(Collins et al., 1993; Collins et al., 1991), human parainfluenza virus3 (Dimock and Collins, 1993), rabies virus (RV) (Conzelmann and Schnell,1994) and MV (Sidhu et al., 1995).

In all these systems, the essential helper proteins were provided eitherby the homologous viruses or by the vaccinia vector vTF7-3 encodingphage T7 RNA polymerase (Fuerst et al., 1986) to drive T7-specifictranscription of transfected plasmids encoding the required proteins N,P and L as pioneered by Pattnaik et al., (1990). These investigationsusing mini-replicons have allowed important insights into the noncodingregulatory regions of the corresponding viral genomes and antigenomes(for a recent discussion see Wertz et al., 1994). Adopting the sameexperimental set up, the rescue of VSV, as RV a member of theRhabdoviridae, has now also been reported (Lawson et al., 1995).

An important drawback of that method (as well as the method reported forthe rescue of negative-strand RNA viruses with a segmented genome) isthe involvement of a helper virus which has to be separated from therescued virus and which can interfere with the replication of the virusto be rescued. For RV and VSV, both belonging to the rigidly structuredRhabdoviridae and replicating to high titers, this is not an importantproblem. However, in case of loosely structured, polymorphic virionstypical for the members of the family Paramyxoviridae and in case ofviruses yielding only relatively low titers, the presence of a helpervirus would render the recovery of rescued viruses difficult and maywell preclude their rescue altogether.

Accordingly, the technical problem underlying the present invention wasto provide genetic material useful for the generation of non-segmentednegative-strand RNA viruses, preferably of the family Paramyxoviridaeand most preferably of measles virus and a system for the recovery ofsuch viruses with reasonable efficiency. The solution to said technicalproblem is provided by the embodiments characterised in the claims.

Thus the present invention relates to a cDNA molecule for the productionof negative-strand RNA virus comprising

-   -   (a) the entire (+)-strand sequence of a non-segmented        negative-strand RNA virus of the family Paramyxoviridae from        which anti-genomic RNA transcripts bearing the authentic        3′-termini can be transcribed; operatively linked to    -   (b) an expression control sequence.

Accordingly, the present invention relates to a cDNA molecule for theproduction of any negative-strand RNA virus of the familyParamyxoviridae. Preferably said anti-genomic RNA transcripts also bearthe authentic 5′-termini.

As has been further found in accordance with the present invention,effective production of measles virus which is a negative-strand RNAvirus of the family Paramyxoviridae, is only obtained if the repliconsspecified by said cDNA molecule consist of an integral multiple of sixnucleotides. This phenomenon will also be referred to as the “rule ofsix” throughout this application. The cDNA molecules of the presentinvention can conveniently be used for the rescue of negative strand RNAviruses of the family Paramyxoviridae.

In a preferred embodiment of the present invention, in said cDNAmolecule, the expression control sequence (b) is an RNA polymerasepromoter.

The present invention further relates to a plasmid containing the cDNAmolecule of the invention. The plasmid of the present invention iscapable of propagation and preferably also expressing the cDNA moleculeof the invention as an antigenomic RNA.

In a preferred embodiment, said plasmid contains an expressible DNAfragment which replaces a preferably homologous DNA region of said cDNAmolecule, or provides additional genetic information.

As was also found in accordance with the present invention, in the caseof MV-based replicons the rule of six must be obeyed, if aforeign—homologous or heterologous—expressible DNA fragment is insertedinto the plasmid containing the cDNA of the invention. In other words,any newly created replicon specified by appropriately constructed cDNAmolecules will only be capable of yielding reasonable amounts of thedesired product, if it obeys the rule of six.

In a most preferred embodiment, said plasmid is characterised in thatthe expressible DNA fragment is inserted into or adjacent to a region ofsaid cDNA encoding a viral protein, said insertion being effected in amanner maintaining the reading frame to create a fusion protein andpermitting the expression of said DNA fragment under the control of thesignal sequences of said viral protein. In accordance with the presentinvention it is anticipated that in various cases appropriate C-terminalextensions of viral proteins will not interfere with theirfunctionality.

In variation to the above described preferred embodiment and alsocomprised by the present invention, the expressible DNA fragment isexpressed in such a manner downstream of a viral protein coding regionto avoid formation of a fusion protein, but nevertheless allowingexpression of the downstream coding sequence either by a stop/restartmechanism where the last A residue of the upstream termination triplettcoincides with that of the start codon of the downstream coding region,or by placing an internal ribosome entry site (IRES) between the twocoding regions; see example 12, second paragraph.

In a further most preferred embodiment, said plasmid is characterised inthat the expressible DNA fragment is inserted into a non-coding regionof said cDNA and flanked by viral signal sequences or heterologoussignal sequences controlling the expression of the RNA fragmentspecified by said DNA fragment; see example 12, first paragraph.

Most preferably, the expressible DNA fragment is placed upstream of theN gene. As has been found in accordance with the present invention, thepositioning of said expressible DNA fragment at the 5′ end of the MVsequence results in a particularly strong expression thereof; see alsoExample 14.

Examples of this embodiment, creating additional transcription units,are provided by the plasmids specifying MVs expressing the heterologousCAT reading frame shown in FIG. 10.

A further preferred embodiment of the invention relates to a plasmidcomprising a genomic ribozyme sequence immediately adjacent to the 3′terminal nucleotide of said cDNA molecule and optionally downstream ofsaid genomic ribozyme sequence at least one terminator, preferably theT7 terminator.

The inclusion of a ribozyme sequence into the plasmid of the inventionleads to the faithful cleavage of the RNA transcript, thus greatlyenhancing the yield of transcripts bearing the correct 3′ termini which,in the case of MV, must obey the rule of six.

The person skilled in the art is, naturally, capable of devising othermeans that result in the generation of the authentic 3′ termini. Suchmeans include the use or incorporation of restriction sites at the DNAlevel, or of tripplehelical DNAs.

In a most preferred embodiment of the plasmid of the invention saidgenomic ribozyme sequence is the hepatitis delta virus genomic ribozymesequence.

The invention relates in a further preferred embodiment to a plasmidbearing said cDNA which is capable of replicating in a prokaryotic host.A preferred example of such a prokaryotic host is E. coli. Illustrationsof this preferred example are all cDNA constructs giving rise tomodified MVs as shown in FIGS. 2 and 10 demonstrating plasmidsreplicating to high copy number in E. coli.

Additionally, the present invention relates in a preferred embodiment toa plasmid bearing said cDNA(s) which is capable of replicating in aeukaryotic host.

The invention envisages the replication and expression (i.e.transcription, followed by translation of the transcripts formed) of therescued vector, i.e. the packaged RNA particles (RNPs), in any suitableeukaryotic, preferably vertebrate, host cell. Preferred host cells arethose with a high replication and expression capacity. Most preferredare those host cells that allow an easy recovery of rescued viruses forfurther replication and subsequent formulation in vaccines.

The invention relates in another preferred embodiment to a plasmidwherein said expressible DNA fragment is a DNA fragment being homologousor heterologous with respect to the negative-strand RNA virus andencoding at least one immunogenic epitope.

In a further preferred embodiment of the present invention in saidplasmid said expressible DNA fragment encodes at least one immunogenicepitope of at least one pathogen, preferably an envelope protein, atleast one gene product lacking in genetically defective individuals ortoxic for targeted malignant cells.

This most preferred embodiment of the invention allows for theconstruction of plasmids as a basis for vaccines that effectively inducean immune response against one or preferably various differentpathogens. In the case that the expressible DNA fragment encodes anenvelope protein of a different virus than measles virus or of anotherpathogen, a measles virus based plasmid can be used to target specificcell types usually not recognised by measles virus. Said cell types canthen selectively be targeted by rescued viruses specified by the plasmidof the invention and confer to said cell type, for example, a moleculethat said cell type is in need of or a toxin, if said cell type is to beeliminated. Naturally, said molecule or toxin is also to be encoded bysaid plasmid. The person skilled in the art is capable of devisingfurther applications of this basic principle for which the plasmid ofthe invention can be used.

Also, said plasmid can encode a product lacking in genetically defectiveindividuals. The rescued virus can then be used for gene therapy of saidgenetically defective individuals.

Further, malignant cells can be targeted by the rescued virus which isbased on the plasmid of the invention and molecules toxic for saidmalignant cells may be delivered.

In a further most preferred embodiment of the present invention, in saidplasmid said expressible DNA fragment is derived from a virus, abacterium, or a parasite.

A further preferred embodiment of the invention relates to a plasmidwherein said expressible DNA fragment encodes an immunogenic epitopebeing capable of eliciting a protective immune response.

In a further preferred embodiment, the cDNA molecule or the plasmidsaccording to the invention are based on an RNA virus which is measlesvirus or mumps virus.

The invention relates further to a prokaryotic or eukaryotic host celltransformed with a plasmid according to the invention. Preferred hostcells have been discussed above.

Additionally, the invention relates to a helper cell capable ofexpressing an RNA replicon from a cDNA molecule of the invention, saidcDNA molecule being comprised in the plasmid of the invention or aplasmid comprising a cDNA molecule for the production of negative-strandRNA virus of a family of the order Mononegavirales which is not a memberof the family of the Paramyxoviridae, said cDNA molecule comprising theentire (+)-strand sequence, operatively linked to an expression controlsequence, and optionally an expressible DNA fragment which replaces apreferably homologous DNA region of said cDNA molecule or providesadditional genetic information, said expressible DNA fragment encodingpreferably at least one immunogenic epitope of at least one pathogen,which most preferably is capable of eliciting a protective immuneresponse, said cell further being capable of expressing proteinsnecessary for transcription, encapsidation and replication of said RNA.

Apart from the features described above, the cDNA molecule for theproduction of negative-strand RNA virus of a family of the orderMononegavirales which is not a member of the family of theParamyxoviridae may also have in certain embodiments the characteristicsof the cDNA molecules of the invention that were discussed herein above,optionally in conjunction with the plasmids of the invention.

In view of the problems the prior art was confronted with for rescuingnon-segmented negative-strand RNA viruses, in accordance with thepresent invention paradigmatic cell lines providing as helper functionsT7 RNA polymerase and MV N and P protein were developed. Rescue of MVscan be directly monitored after transfection with plasmids specifyingantigenomic RNAs and MV L mRNA. In principle, analogous helper celllines can be generated for any of these viruses; thus this rescueapproach is applicable for all Mononegavirales replicating in vertebratecells.

Thus, in a preferred embodiment of the helper cell according to theinvention said proteins necessary for encapsidation, transcription andreplication of said RNA are an RNA polymerase, preferably T7 RNApolymerase and optionally T3 RNA polymerase, and N and P protein,preferably of the virus to be rescued. In accordance with the presentinvention, said proteins are expressed from stably transfectedexpression plasmids, henceforth defined as genomic expression.

Since the rescue system now developed, in contrast to the one used forrescue of RV (Schnell et al., 1994), VSV (Lawson et al., 1995) and veryrecently also for SeV (D. Kolakofsky, personal communication), does notrely on any helper virus, there is no need to separate the rescued virusfrom the vast excess of any helper virus. Elimination of vaccinia virusfrom rescued virus is accomplished by a simple filtration step in thecase of the rigidly structured virions of Rhabdoviridae but wouldinvolve more complex purification schemes in case of pleomorphicParamyxoviridae, particularly those not replicating to high titers suchas MV. Furthermore, for viruses impaired in replication and/or buddingby the vaccinia virus, rescue using the prior art systems might failaltogether. Another possible drawback of the prior art systems based onthe vaccinia helper virus is the high frequency of DNA recombinationsoccurring in the cytoplasm of vaccinia virus infected cells which mightcause recombination of the plasmid bearing the anti-genomic sequencewith the plasmids encoding N, P and L protein required for the helperfunction; this may lead to rescue of viruses containing N, P and Lsequences derived in part from the helper plasmids rather than from theplasmid bearing the antigenomic sequence. The helper cell systemcircumvents all of these problems and should in principle be applicablefor the rescue of any of the Mononegavirales replicating in vertebratecells.

It may not be necessary for the rescue of any single representative ofMononegavirales, to establish a helper cell line expressing the cognateN and P protein (in addition to T7 polymerase). Mini-replicon constructscontaining the noncoding terminal regions (NCTs) of canine distempervirus (CDV) which is like MV a morbillivirus, differing from MV in 35%of the nucleotides in the NCTs, replicate in the MV-specific helpercells at an efficiency approaching that of the homologous MVmini-replicon. Thus, possibly CDV could be rescued with the 293-3-46cells, which were developed in accordance with the present invention andmore generally, any helper cell line might be able to rescue a number ofnot too distantly related Mononegavirales. This will probably depend onthe compatibility of the proteins elicited by the related viruses, whichwas shown not to be the case for SeV-specific N and P and PIV3-specificL (Curran and Kolakofsky, 1991).

For the establishment of new helper cell lines for other viruses whichare also envisaged by the present invention, the followingconsiderations might be helpful. The constitutive expression of the T7RNA polymerase and the MV proteins N and P did not impair the long termstability of the 293-3-46 cell line, as mentioned in the examplesattached hereto. Thus, inducible expression of these proteins, forexample, by the approaches described by the group of Bujard (for areview see Gossen et al., 1993) will probably not be necessary, althoughit cannot be excluded that the N and P proteins of other viruses aremore deleterious for cell growth than those of MV. Titration of theplasmids used for transfection proved essential, showing that a ratio ofabout 1:1000 of L-encoding and antigenome-producing plasmid,respectively, was optimal, in agreement with the deleterious effect ofhigh VSV L expression for VSV replication noted by Schubert et al.(1985). An alternative mode of transiently supplying L, using a plasmidcontaining a CMV promoter/enhancer and an intron upstream rather thandownstream of the L coding region to permit some export of the long LmRNA from the nucleus, was also successful in rescue, but the efficiencywas not better than with the standard method of cytoplasmic T7-dependentL expression and more than a hundred times more L encoding plasmid wasoptimal for rescue. In view of these experiences, the decision not toinclude an L encoding plasmid for the generation of helper cells, thusallowing expression of L at adjustable ratios, was probablyadvantageous. Nevertheless, it should be mentioned that a cell linestably expressing SeV-derived N, P and L which mediates long termreplication of natural SeV DIs has been described (Willenbrink andNeubert, 1994). It is important to note that this cell line differsfundamentally from the helper cells defined in the present invention byits lack of T7 polymerase. As a consequence, no rescue of a virus andnot even of a minireplicon from cloned DNA is feasible with this cellline.

In a further preferred embodiment of said helper cell said cell istransfected with at least one of said above described plasmids, saidplasmids containing variant antigenomic cDNA of a representative of theMononegavirales, and is additionally stably transfected with a plasmidcomprising DNA encoding the cognate viral L protein.

Thus, instead of selecting for a helper cell that also encodes per sethe viral polymerase (L protein), said L protein is transfected intosaid helper cell on a different plasmid, preferably by cotransfection.Further, a skilled person using the teachings of the present inventionis able to create a suitable helper cell line expression also L protein,in which case cotransfection is not necessary.

In a most preferred embodiment of said helper cell, the genes encodingsaid N, P and L proteins are derived from measles or mumps virus.

In a further most preferred embodiment said helper cell is derived fromthe human embryonic kidney cell line 293 (ATCC CRL 1573). A preferredexample of such a cell is clone 293-3-46 described in the examples.

The invention further relates to an infectious negative-strand RNA virusstrain belonging to the order Mononegavirales isolated from the helpercell of the invention.

It must be recalled that five years ago, in an erroneous account, MVrescue was reported by our laboratory (Ballart et al., 1990 and EP-A 0440 219), using the same basic principle. At that time, the experimentswere based on microinjection of initiation complexes, consisting of T7RNA polymerase and plasmids specifying MV genomes or antigenomes, into aparticular cell line containing defective but replicating MV genomes.However, the rescue by microinjection experiments, unfortunately carriedout by only one collaborator, could not be repeated, and all purportedlyrescued viruses did not contain the genetic tag, as summarised in acommentary to these extremely sad and devastating events (Aldhous,1992). It is now clear that rescue of MV could not be expected with thatexperimental setup for several reasons, in particular due to additionalnucleotides at both ends of the generated RNAs and due to a cloningmistake rendering the RNA incompatible with the rule of six (Calain andRoux, 1993; the present invention).

The rescue efficiency, in comparison to rescue of positive-strand RNAviruses (Perrotta and Been, 1990), is low, since only 1 to 6 out of 10⁶transfected cells, each exposed on average to about 2.5×10⁵ molecules ofantigenomic and 80 to 800 molecules of L-encoding plasmid, trigger theformation of syncytia. Nevertheless, in comparison with the rescuemethod described for RV and VSV, where about 2×10⁷ cells are transfectedto obtain one rescue event (Lawson et al., 1995; Schnell et al., 1994),the MV rescue compares well, particularly in view of the fact that theMV genome size is roughly 4.5 kb larger and thus in principle moredifficult to rescue. Importantly, the low efficiency should notconstitute a difficulty for the rescue of MV variants replicating onlyto titer levels even orders of magnitude lower than the Edmonston Bstrains, since the bottle-neck of rescue is constituted most likely byan early event. It is important to note that on cells fixed at varioustimes after transfection, immunofluorescence indicating H or M geneexpression was monitored exclusively in syncytia and there was noindication that rescue was confined to single cells. When rescue isvisible directly by syncytia formation, already thousand of progeny MVgenomes have arisen; impaired and thus slowly replicating virus variantsmight not form visible syncytia initially, but should be revealed aftersplitting of the transfected cell culture or upon seeding onto freshVero cells.

The invention further relates to a method for the production of aninfectious negative-strand RNA virus belonging to the orderMononegavirales, comprising the steps of

-   (a) transfecting the helper cell of the invention with any one of    the plasmids described above and comprising antigenomic DNA from a    virus belonging to the order Mononegavirales (first vector) and    optionally a plasmid comprising DNA encoding the viral L protein    (second vector); and-   (b) recovering the assembled infectious negative-strand RNA viruses.

Transfection with the second vector is not necessary, if the helper cellgenomically expresses the viral L protein.

In a preferred embodiment of the method of the invention, the ratio ofthe first vector and the second vector is about 1000:1.

In accordance with the present invention it has been shown that theabove ratio is optimal for transfection efficiency.

In further preferred embodiments of the method of the invention, saidrecovery is either directly effected from the transfected helper cellculture after syncytia formation or, after mixing of detached helpercells with any other cells competent of being infected and replicatingthe assembled RNA viruses.

The invention relates further to a vaccine comprising the RNA virusaccording to the invention which optionally is obtainable by the methodof the invention described above, optionally in combination with apharmaceutically acceptable carrier.

The advantages of the vaccine of the present invention will be brieflydiscussed below.

In the past, a variety of DNA viruses and positive-strand RNA viruseshave been used as carriers to direct the expression of heterologousgenes or gene segments in host cells, mainly with the aim to elicitimmune protection against the pathogen from which the heterologousgenetic material was derived. The main advantage of using such livevaccines is their ability to multiply and typically infect a variety ofdifferent cell types, generating the antigens of interestintracellularly which can therefore be presented efficiently to theimmune system, thus facilitating the induction of both T cell help andcytotoxicity. In contrast, killed vaccines or proteins manufactured byrecombinant DNA technology are much less efficient, even byadministration in various particulate forms developed recently, whichare more efficient than traditionally used adjuvants. In addition, suchvaccines typically induce no mucosal immunity, which is very importantfor protection against pathogens entering by the respiratory orintestinal route. Failure to induce mucosal immunity is also typical forthe immunisation approach using injection of naked DNA encodingantigens.

On the other hand, most replicating vaccines constitute a possiblethreat, even if they are not proliferating, such as avipox vectors inhumans (Baxby and Paoletti, 1992). Complex viral vectors (e.g. based onvaccinia virus and related pox viruses, adenoviruses or herpesviruses)and bacterial vectors (e.g. based on derivatives of the agents causingtuberculosis or cholera) inherently elicit many lateral, unnecessaryand/or undesired immune responses. In addition, DNA integration in thegenome of infected or transfected cells bears at least the potential formalignant transformation. Multiauthored assessments of various types ofvaccines have been published recently (vaccines and public health;Internat. J. of techn. Ass. in Health care 10, 1-196 1994; Science 265,1371-1451, 1994), from which the particular benefits of small RNA-basedlive vaccines are evident.

Several engineered positive-strand RNA viruses have been described forpotential use as vectors for immunisation purposes; early examplesinclude poliovirus (Burke et al., 1988) and Sindbis virus (Xiong et al.,1989) and among several more recent accounts, involving largerpolypeptide fragments expressed from various representatives of thePicornaviridae, just one should be mentioned here (Andino et al., 1994).

However, it must be stressed that the use of RNA viruses as vectors forvaccination purposes crucially depends on the stability of the foreigngenetic material during the replication of the virus. This is not atrivial problem, because these viruses rely on a polymerase devoid ofproofreading activity. Said problem has advantageously been solved bythe present invention: in comparison to vaccine vectors based onpositive-strand RNA viruses as mentioned above, the vaccine of theinvention as exemplified by MV-based di- or multivalent vaccines showseveral important advantages which are valid in principle for any othermember of the Paramyxoviridae such as mumps virus. First, the size ofinserts is not a priori limited by a requirement to fit into anicosahedral protein shell. Second, the tight encapsidation of thegenomes of Mononegavirales obviates RNA secondary structure which isvery important in case of the positive-strand RNA viruses over the wholegenome length to allow proper replication without annealing of theproduct to the template RNA strand; RNA segments encoding foreignantigens are not evolved to meet such requirements. Third, due to themodular set up of the genome, different insertion sites and expressionmodes, either as additional transcription units or as elongation ofexisting transcription units, expressing the inserted downstream readingframes by stop/restart or by an internal ribosome entry site can beenvisaged, thus allowing a large range of different expression levelsaccording to the position within the MV transcription gradient. Fourth,due to extremely low recombination frequencies, Mononegavirales can beexpected to retain nonessential genetic material much more stably thanpositive-strand RNA-viruses. Finally, the rule of six, valid for MV aswas found in accordance with the present invention and for otherParamyxovirinae (Calain and Roux, 1993), but as judged from cognatemini- and midi-replicons, not for Rhabdoviridae (Conzelmann and Schnell,1994) or for Pneumovirinae (Collins et al., 1993), should even increasethe faithful retention of foreign coding regions inserted inParamyxovirinae in comparison to other Mononegavirales. Such anadditional genetic stability can be anticipated because only one in sixadventitiously arising large deletions and no small insertion ordeletion of 1 to 5 nucleotides in a region nonessential for viralreplication are expected to lead to viable progeny.

Further, knowledge of the nucleotide sequence variants conferringattenuation will allow one to change the coding sequences not implicatedin attenuating properties according to the evolution of the viruses overthe years thus permitting one to “update” the vaccines without incurringthe danger of losing the quality of attenuation.

The invention additionally relates to the use of the plasmid of theinvention in somatic gene therapy.

Since viral envelope proteins can be exchanged among differentrepresentatives of Mononegavirales, as shown here by the replacement ofthe MV envelope proteins with the VSV glycoprotein, it seems feasible totarget the replion based on the replication machinery of Mononegaviralesto particular cell types; thus, certain applications in somatic genetherapy can be envisaged. Advantages in comparison to existing vectorsfor gene therapy include their small size, thus limiting antigenreactions to a few proteins, and their complete inability to integrateinto DNA and thus to transform cells.

Additionally, the invention relates to the use of the plasmid of theinvention for targeting special cell types. An outline of such targetingschemes and applications has been provided above.

The invention relates further to the use of the plasmid of the inventionfor the functional appraisal of mutations found typically in MV variantsresponsible for fatal subacute sclerosing panencephalits or for theidentification of mutations responsible for attenuation ofParamyxoviridae strains, preferably measles virus strains.

Finally, the invention relates to a diagnostic composition comprising atleast one cDNA molecule of the invention and/or at least one plasmid ofthe invention.

THE FIGURES SHOW

FIG. 1: Genomic map of measles virus

FIG. 2: Plasmid vectors specifying RNAs with correct MV-specifictermini. The numbers below the plasmid names indicate the length innucleotides of the RNAs generated after ribozyme self-cleavage. Genomicor antigenomic sense of the specified RNAs is indicated by (−) and (+),respectively. Note that the MV nucleotide sequences present in theseplasmids deviate in 30 positions from EMBL accession No K01711, mostnotably by a deletion of an A residue at pos. 30, compensated byinsertion of an A at pos. 3402. For a commented overview of a MVconsensus sequence see Radecke and Billeter (1995).

FIG. 3: Western blot showing the expression of MV N and P proteins inMV-infected 293 cells, uninfected 293 cells and in cell line clones293-3-46 and 293-3-64, respectively. Arrows indicate the position of thestructural MV N and P proteins as well as the nonstructural V proteinarising from MV P gene transcript editing.

FIG. 4: Overview of experimental components and procedures for rescue.A: Mini-replicon rescue, implicating transfection of in vitrotranscribed RNA and coinfection with MV, supplying helper proteins N, Pand L (and for later stages also M, F and H, as well as nonstructuralproteins C and V). B: MV rescue, implicating transfection of plasmidDNAs into helper cells mediating both artificial T7 transcription and Nand P functions. For explanation of most symbols see FIG. 2. The Lencoding plasmid pEMC-La contains an internal ribosome entry sitederived from encephalomyocarditis virus (stippled oval, EMC IRES), fusedto the L coding region such that the initiator AUG of EMCV and Lcoincide; a poly dA tract downstream (about 40 dAs) is indicated as pdA.These two devices ensure transcript stability as well as efficienttranslation from the transcripts generated in the cytoplasm.

FIG. 5: Assay of CAT activity elicited in 293-3-46 helper cells bytransfection of the plasmid constructs p107MV(−):CAT and p107MV(−):CAT,specifying mini-replicons, and construct p(+)NP:CAT, specifying amidi-replicon. The backbone of the plasmid pT7P2lacZ is similar asdescribed in Pelletier and Sonenberg (1988). The CAT reading frame ofthe original plasmid is replaced by the lacZ reading frame.

FIG. 6: Visualisation of syncytia formed in 293-3-46 helper cells. A:Rescue experiment, viewed by phase contrast microscopy 4 days aftertransfection. B, C: Cells grown on glass cover slips, fixed 3 days aftertransfection and viewed by phase contrast (B) or indirectimmunofluorescence microscopy using a monoclonal antibody directedagainst MV M protein (C). Similar results were obtained with an antibodyagainst H. The bar length represents 100 μm.

FIG. 7: Sequence determination of plaque-purified viruses, carried outby RT-PCR followed by cycle sequencing as described in the Examples. Theleft lanes of the relevant area reproduced from a sequencing gel relateto our laboratory Edmonston B strain, the right lanes to the rescuedvirus. Nucleotide positions indicated correspond to those in the MVconsensus sequence as defined in FIG. 2.

FIG. 8: Replication behaviour of plaque-purified viruses, evaluated byan overlay technique as described in the Examples. The derivatives ofrescue experiments, the standard MV tag EdB and the 504 nucleotidedeletion mutant MVA5F EdB are compared with a clone from our laboratoryEdmonston B virus strain. The results of two independent experimentsusing a representative clone of each virus species are shown.

FIG. 9: Northern blots revealing mRNAs of the rescued MV derived fromp(+)MV, and the MV deletion mutant derived from p(+)MVΔ5F (FIG. 2). Themonocistronic F, M and H mRNA species (open triangles) and thebicistronic MF and FH mRNAs (black triangles) are revealed by M, F, andH-specific probes. The F-specific mono- and bicistronic RNAs induced bythe deletion mutant are clearly smaller than the corresponding RNAsinduced by the rescued standard MV (ΔF, 1869 rather than 2372 nt.calculated, without considering poly A tails; MΔF, 3338 rather than 3842nt., and ΔFH, 3830 rather than 4334 nt.).

FIG. 10( a) Plasmids for production of standard and deleted MVs andhybrid MVs containing additional genes or exchanged envelope proteins.

-   -   Note that two MV chimeric clones recovered from p(+)MPCATV and        from p(+)MHCATV after 10 cycles of infection still expressed CAT        activity encoded by the additional transcription unit in every        one of the 10 clones taken from the tenth cycle tested.

FIG. 10B-1 Plasmids for production of standard and variant Edmonston Bmeasles viruses

-   -   p(+)MV: The RNA polymerase provides anti-genomic MV RNA with two        sequence tags in positions 1702 (A) and 1805 (AG)    -   p(+)MV C⁻: The antigenomic RNA corresponds to that obtainable        from p(+)MV with the exception that the C-protein ORF is        rendered non-functional by the introduction of two point        mutations in positions 1830 (C) and 1845 (A).    -   p(+) MV V⁻: The antigenomic RNA corresponds to that obtainable        from p(+)MV with the exception that the V protein ORF is        rendered non-functional by mutating the conserved “editing        site”.    -   p(+)MV ΔM: The antigenomic RNA corresponds to that obtainable        from p(+)MV with the exception that the complete ORF of the M        gene (Δ320 amino acids) with the exception of 15 amino acids has        been deleted.    -   p(+)MV Δ5F: The antigenomic RNA corresponds to that obtainable        from p(+)MV with the exception that a deletion of 504        nucleotides (nucleotides 4926-5429 are missing) has been        introduced into the F gene.    -   p(+)MV FΔcyt: The antigenomic RNA corresponds to that obtainable        from p(+)MV with the exception that the sequence encoding the        cytoplasmic part of the F protein has been exchanged by a        different fragment encoding the cytoplasmic part of the F        protein derived from a SSPE case. A premature stop codon results        in a deletion mutant having a deletion in the F protein        cytoplasmic domain.    -   p(+)MV Fxc SeV: The antigenomic RNA corresponds to that        obtainable from p(+)MV with the exception that the sequence        encoding the cytoplasmic domain of the F protein has been        replaced by the corresponding sequence from Sendai virus.    -   p(+)MV HΔcyt: The antigenomic RNA corresponds to that obtainable        from p(+)MV with the exception that the sequence encoding the        cytoplasmic domain of the H protein has been replaced by a        fragment carrying a deletion.

FIG. 10-B2 Plasmids for production of Edmonston B measles virus chimerasand vectors.

-   -   p(+) MGFPNV: The antigenomic RNA corresponds to that obtainable        from p(+)MV with the exception that an additional cistron has        been incorporated upstream of the MV N-ORF that allows MV        dependent expression of the reporter gene encoding green        fluorescent protein; said ORF is inserted into a multiple        cloning site.    -   p(+)MPCATV: The antigenomic RNA corresponds to that obtainable        from p(+)MV with the exception that an additional cistron has        been incorporated downstream of the P ORF allowing the        expression of the CAT gene.    -   p(+) MPGFPV: The construct corresponds to p(+)MPCATV with the        exception that the CAT coding sequence has been replaced by the        GFP coding sequence which is, again, cloned into a multiple        cloning site.    -   p(+)MHCATV: The antigenomic RNA corresponds to that obtainable        from p(+)MV with the exception that an additional cistron has        been inserted downstream of the H ORF which allows the        expression of the CAT gene.    -   p(+)MG/FV: The antigenomic RNA corresponds to that obtainable        from p(+)MV with the exception that the MV F and H genes have        been replaced by a gene encoding an VSV G protein, the        cytoplasmic part of which has been replaced by the cytoplasmic        part of the MV F protein.    -   p(+)MGV: The antigenomic RNA corresponds to that obtainable from        p(+)MV with the exception that the MV F and H genes have been        replaced by a gene encoding an VSV G protein.

FIG. 11: Electron microscopy of BHK cells infected with replicatingagent rescued from p(+)MGV.

-   -   Large arrays of RNPs typical for MV-infected cells are visible,        showing unimpaired replication capability of the chimeric viral        RNA.

FIG. 12: Electron microscopy of BHK cells infected with replicatingagent rescued from p(+)MGV.

-   -   Pleomorphic particles resembling MV virions are formed despite        the fact that in these infected cell cultures exclusively VSV G        protein and no trace of the MV envelope proteins F and H was        detectable by Western blotting.

FIG. 13: Electron microscopy of BHK cells injected with VSV: VSV virionparticles.

-   -   The typical bullet-shaped VSV virions differ completely from the        pleomorphic MV-like particles shown in FIG. 12.

The examples illustrate the invention:

EXAMPLE 1 Cells and Viruses

Cells were maintained as monolayers in Dulbecco's modified Eagle'smedium (DMEM) supplemented with 5% foetal calf serum (FCS) for Verocells (African green monkey kidney), with 10% FCS for 293 cells (humanembryonic kidney) and with 10% FCS and 1.2 mg/ml G418 for the stablytransfected 293 derived cell clones.

To grow MV virus stocks reaching titers of about 10⁷ pfu/ml, recombinantviruses were propagated in Vero cells, and the vaccine strain EdmonstonB was grown in Vero or 293 cells. One round plaque-purification wascarried out by transferring a syncytium to a 35 mm Vero cell culturewhich was expanded to a 175 cm² dish. Virus stocks were made from 175cm² cultures when syncytia formation was pronounced. Cells were scrapedinto 3 ml of OptiMEM I (GIBCO BRL) followed by one round of freezing andthawing. The virus titrations were carried out on 35 mm Vero cellcultures. After 2-3 h of virus adsorption, the inoculum was removed andthe cells were overlaid with 2 ml of DMEM containing 5% FCS and 1%SeaPlaque agarose. After 4-5 days, cultures were fixed with 1 ml of 10%TCA for 1 h, then UV-cross linked for 30 min. After removal of theagarose overlay, cell monolayers were stained with crystal violetdissolved in 4% ethanol, and the plaques were counted.

EXAMPLE 2 Generation of Cell Line 293-3-46

Before the transfection, all plasmids were linearized by digestion withSfiI and sterilised by ethanol precipitation. Cells were seeded into one35 mm well for transfection during 13 h as described below. Thetransfection mix contained 5 μg of pSC6-N, 4 μg of pSC6-P, and 1 μg ofpSC6-T7-NEO. Then, cells were washed once with 2 ml of phosphatebuffered saline (PBS; 137 mM NaCl, 2.7 mM KCl, 8 mM Na₂HPO₄, 1.5 mMKH₂PO₄), and DMEM containing 10% FCS was added. After 2 days in culture,the cells of the 35 mm well were splitted to two 75 cm² dishes, andselection under 1.2 mg/ml G418 was started changing the medium everysecond day. After ˜2 weeks, the first clones of a total of ˜100 cloneswere transferred to 5 mm wells. When a clone had expanded to a 21 mm- or35 mm well, cells were seeded for screening. The expression of the MV Nand P proteins was analysed by Western blotting (see also below) using˜⅓ to 1/10 of the total lysate of a confluent 21 mm well. To monitor thefunctionality of the T7 RNA polymerase, a 35 mm cell culture wastransfected with 4 μg of pEMC-Luc (Deng et al., 1991), and theluciferase activity in 1/125 of the cleared total lysate (Promegaprotocol; harvest 1 day after transfection) was measured in aluminometer. Clones expressing the MV N and P proteins comparable to thesame number of 293 cells infected with MV and showing a T7 RNApolymerase activity as high as possible were chosen to test theirperformance in allowing MV DI RNAs to express. CAT. Here, 5 μg of theplasmids p107MV(+):CAT, p107MV(−):CAT, or p(+)NP:CAT with or without 100ng of pEMC-La were transfected. After 1 day, cells were lysed, and ¼ ofthe cleared lysates was tested for CAT activity.

EXAMPLE 3 Plasmid Constructions

All cloning procedures were basically as described in Sambrook et al.(1989). PCR amplifications were carried out using the proofreading PfuDNA polymerase (Stratagene) and primers with a 3′ terminalphosphorothioate bond instead of a phosphodiester bond (Skerra, 1992).DNA sequences of the synthetic oligonucleotides are given in lower casefor non-MV nucleotides and in upper case for the MV nucleotides;sequences of relevant restriction endonuclease recognition sites areunderlined. The construction of the plasmid p107MV (−):CAT can be foundin Sidhu et al., 1995. Plasmid p107MV(+):CAT is the analogue of theplasmid p107MV(−):CAT. The additional intercistronic region of p(+)NP:CAT that is similar to the NP intergenic boundary was constructed byinserting(5′-ctaGCCTACCCTCCATCATTGTTATAAAAAACTTAGGAACCAGGTCCACACAGCCGCCAGCCCATCAACgCgcgtatcgcgata-3′, SEQ ID NO:1, MV(+) 1717-1782) and the internallycomplementary oligonucleotide into the SpeI site of the P gene. ThePCR-amplified CAT coding region was inserted as depicted in FIG. 2.

The description of the assembly of the first MV full length DNA, thesource of MV nucleotides 2044-14937 in later versions of full lengthclones such as peuT7MV(−) (see below), is given in Ballart et al., 1990.The main features of the plasmid p(+) MV (FIG. 2) are as follows: The T7promoter allows the synthesis of the My antigenomic RNA preciselystarting with the first nucleotide. The genomic hepatitis delta virusribozyme (δ) liberates upon self-cleavage the correct MV 3′ terminalnucleotide. Directly downstream of the δ ribozyme, the T7 RNA polymeraseterminator Tφ stops most of the transcribing polymerases. This ensuresthat adjacent sequences derived from the vector backbone will notinterfere with the cleavage activity. The cloning of p(+)MV started byannealing two internally complementary oligonucleotides #191(5′-ggggaaccatcgatggataagaatgcggccgcaggtac-3′ SEQ ID NO:2) and #192(5′-ctgcggccgcattcttatccatcgatggttccccgc-3′ SEQ ID NO:3) yielding ashort polylinker that carries the restriction sites for SaclI, ClaI,NotI, and KpnI. This new polylinker replaced the SaclI-KpnI fragment inpBloT7 derived from pBluescript KS(+) (Stratagene) containing the T7promoter fused to a NsiI site (Kaelin, 1989) thus forming the plasmidpBloT7NSCNK. To clone in the 5′-terminal 2041 by of the MV antigenome(up to the SaclI site), a NsiI-digestion was followed by treatment withKlenow polymerase in the presence of all four dNTPs. This created ablunt-end cloning site flush to the nontranscribed part of the T7promoter sequence. A MV fragment comprising the nucleotides 1-2078 wasgenerated from the 3351 by PvuI-fragment of peuMV(−) by PCRamplification using primers #182 (5′-ACCAAACAAAGTTGGGTAAGGATAG-3′, SEQID NO:4, MV (+) 1-25), and #183 (5′-CAGCGTCGTCATCGCTCTCTCC-3′, SEQ IDNO:5, MV(−) 2077-2056). Note that the additional A residue at positionMV(+) 30 (Sidhu et al., 1995) derived from the MV sequence of peuMV(−)was later deleted by mutational PCR. Upon SaclI-treatment, the MVfragment was ligated into the vector to yield pT7MV(+) 5′. Next, the3′-terminus of the antigenome was linked to the sequence of 8 followeddownstream by T4). The MV 3′-fragment (nucleotides 14907-15894) wasgenerated from the 14046 by PvuI-fragment of peuMV(−) by PCRamplification using the primers #186 (5′ GAGAAGCTAGAGGAATTGGCAGCC-3′,SEQ ID NO:6, MV(+) 14907-14930) and #187(5′-ttctgaagactcACCAGACAAAGCTGGG-3′, SEQ ID NO:7, MV(−) 15894-15879).Another PCR amplification on the plasmid peu3a δTφ with the primers #I84(5′-ataagaatgcggccgcatccggatatagttcctcc-3′, SEQ ID NO:8) and #FR4(5′ttctgaagactcTGGTggccggcatggtcccag-3′, SEQ ID NO:9, MV(+) 15891-15894)yielded the genomic HDV ribozyme linked to the Tφ. Both primers #FR4 and#I87 contain close to their 5′ ends the recognition sequence for BbsIwhich creates a sticky end on both fragments comprising the four3′-terminal MV nucleotides (MV(+) TGGT). After the digestions of the MV3′-fragment with ClaI and BbsI, of the δ/Tφ-fragment with BbsI and NotI,and of pT7MV(+) 5′ with ClaI and NotI, a three-way ligation yielded theplasmid pT7MV(+) 5′ 3′ δTφ. The final step to generate p(+)MV was tofill in the remaining antigenomic MV nucleotides 2044-14937 by athree-way ligation. The SaclI-PacI fragment (MV(+) nucleotides2044-7242) and the PacI-ClaI fragment (MV nucleotides 7243-14937) werereleased from plasmid peuT7MV (−). These two fragments were ligated intopT7MV(+) 5′ 3′ δTφ from which the remaining polylinker (SaclI-ClaI) hadbeen removed. The plasmid p(−)MV (FIG. 2) was constructed similarly. Theself-cleavage activity of δ was demonstrated by detecting the expectedsmall 3′ fragments of in vitro made RNAs on a 5% polyacrylamide/7M ureagel. To generate p(+)MVΔ5F carrying a 504 nt-deletion (MV(+) 4926-5429)in the 5′ noncoding region of the F gene, first a PCR was carried our onplasmid pAeFI (Huber, 1993) using primers #88(5′-CcGAATCAAGACTCATCCAATGTCCATCATGG-3′, SEQ ID NO:10, MV(+) 5430-5461)and #89 (5′-AGAGAGATTGCCCCAATGGATTTGACCG-3′, SEQ ID NO:11, MV(−)5550-5523). The PCR fragment digested with HpaI replaced the NarI-HpaIfragment in pAeF1. The NarI-PacI-fragment of this vector then replacedthe corresponding fragment in p(+)MV.

The vector backbone of pEMC-La is based on pTM1 (Moss et al., 1990) inwhich a NcoI-site overlaps with an ATG trinucleotide. Using this ATG asthe start codon, an open reading frame inserted into this NcoI-site istranslationally controlled by the encephalomyocarditis (EMC) virusinternal ribosome entry site (IRES). The MV L coding sequence linked toan artificial poly(dA)-tract was taken from vector pAeL (Huber, 1993) intwo steps: first, a 405 by fragment containing the MV nucleotides9234-9630 was generated by PCR using primers #194(5′-gtggatccATGGACTCGCTATCTGTCAACC-3′, SEQ ID NO:12, MV(+) 9234-9255)and #195 (5′-AGTTAGTGTCCCTTAAGCATTGGAAAACC-3′, SEQ ID NO:13, MV(−)9360-9602); second, a 6265 by fragment comprising nucleotides 9572-15835of the MV L gene sequence joined to the poly (dA)-tract was excised withEcoRI. After removing the NcoI-EcoRI part of the polylinker in pTM1 anddigesting the PCR fragment also with NcoI and EcoRI, a three-wayligation including the 6265 by EcoRI-fragment yielded pEMC-La.

To eliminate the T7 promoter located 5′ of the CMV promoter/enhance inthe vectors pSC-N and pSC-P (Huber et al., 1991), pSC6-N and pSC6-P wereconstructed by replacing a PvuI-EcoRI fragment with the correspondingfragment of pSP65 (Promega). pSC6-T7 was generated by exchanging the Ngene insert of pSC6-N by the fragment carrying the T7 RNA polymerasegene of pAR 1173 (Davanloo et al., 1984). pSC6-T7-NEO was constructed byligation of the phosphoglycerol kinase promoter-neomycin-resistancecassette (Soriano et al., 1991) into the unique AvrII site of pSC6-T7using appropriate linker oligodeoxyribonucleotides. All cloning siteswere verified by sequencing.

EXAMPLE 4 Transfection of Plasmids and Harvest of Reporter Gene Products

Cells were seeded into a 35 mm well to reach ˜50-70% confluence whenbeing transfected. 3-8 h before transfection, the medium was replacedwith 3 ml of DMEM containing 10% FCS. G418 was omitted henceforthbecause of its toxic effect during transfection. All plasmids wereprepared according to the QIAGEN plasmid preparation kit. The protocolfor the Ca²⁺ phosphate coprecipitation of the DNA was adapted fromRozenblatt et al. (1979). The plasmids (2-10 μg per 35 mm well) werediluted with 300 μl of 1× transfection buffer (137 mM NaCl, 4.96 mM KCl,0.7 mM Na₂HPO₄, 5.5 mM dextrose, 21 mM HEPES pH 7.03). 1 M CaCl₂solution was added to a final Ca²⁺-concentration of 125 mM, and the mixwas incubated at 20° C. for 30-120 min. The coprecipitates were addeddropwise to the culture and the transfection was carried out at 37° C.and 5% CO₂ for ˜15 h. Then, the transfection medium was replaced with 3ml of DMEM containing 10% FCS. The products of the reporter genes wereharvested 24-37 h after transfection. Cells were washed and lysed withReporter lysis buffer (Promega), and CAT and luciferase assays were donefollowing the supplier's protocol.

EXAMPLE 5 Experimental Set-Up to Rescue MV

293-3-46 cells prepared for transfection as described above weretransfected with 5 μg of the plasmid harbouring the MV antigenomic DNAin presence or absence of 1-100 ng of the plasmid specifying the MV LmRNA. First syncytia appeared about 2-3 days after transfection when thecells were still subconfluent. To allow syncytia formation to progressmore easily, almost confluent cell monolayers of each 35 mm well werethen transferred to a 75 cm² dish. When these cultures reachedconfluence, cells were scraped into the medium and subjected once tofreezing and thawing. Cleared supernatants were used to infectmonolayers of Vero cells either to grow virus stocks or to harvest totalRNA for analysis.

EXAMPLE 6 RT-PCR, Cycle Sequencing, Northern Blot, Western Blot,Immunofluorescence

For RT-PCR followed by cycle sequencing, Vero cells were infected withcleared virus suspensions either harvested from rescue cultures or fromlater passages, and total RNA was isolated according to Chomczynski andSacchi (1987). 2 μg of total RNAs were first hybridised with 10 pmol or1 nmol of random hexamer primers by heating to 80° C. for 1 min and thenquick-cooled on ice. Reverse transcriptions were carried out with 200 Uof MMLV-RT (GIBCO BRL) in the presence of 1 mM dNTPs in a buffercontaining 20 mM Tris-HCl pH 8.4, 50 mM KCl, 2.5 mM MgCl₂, 0.1 mg/mlbovine serum albumin, and 1 U RNAsin (Promega). The mixes were kept at20° C. for 10 min, incubated at 42° C. for 1 h, and terminated byheating at 95° C. for 10 min. 1/10 of the reaction volumes was used astemplates for the PCR amplification with the primers #59(5′-ACTCGGTATCACTGCCGAGGATGCAAGGC-3′, SIQ ID NO:14, MV(+) 1256-1284) and#183 (5′-CAGCGTCGTCATCGCTCTCTCC-3′, SEQ ID NO:5, MV(−) 2077-2056). After40 cycles, the 822 by fragments were isolated using the QIAquick gelextraction kit (QIAGEN). The sequencing reactions were done according tothe linear amplification protocol (Adams and Blakesley, 1991). Primer#76 (5′-ctaGCCTACCCTCCATCATTGTTATAAAAAACTTAG-3′, SEQ ID NO.15, MV(+)1717-1749) was used for the tag in the 5′ noncoding region of the P geneand primer #6 (5′-ccggTTATAACAATGATGGAGGG-3′, SEQ ID NO:16, MV(−)1740-1722) for the tag in the 3′ noncoding region of the N gene.

Total cellular RNA for Northern blot analysis was isolated from Verocells using the TRI REAGENT® (Molecular Research Center, Inc.) andpoly(A) RNA was purified using oligo(dT)₂₅-coated super paramagneticpolystyrene beads (Dynal) and a magnetic particle concentrator. The RNAwas electrophoresed through a 1% agarose gel in 6%formaldehyde-containing running buffer and transferred to a Hybond-N⁺membrane (Amersham) by capillary elution in 20×SSC. Filters wereprehybridised at 42° C. for 4 h. Hybridisation was performed overnightat 42° C. in 50% (v/v) formamide, 1 M NaCl, 10% (w/v) dextran sulfate,1% SDS, yeast tRNA (0.1 mg/ml) containing 2×10⁶ c.p.m./ml of an [α-³²P]dATP-labeled DNA probe prepared with Prime-It II (Stratagene). Thefollowing DNA fragments were used for random priming: the 1.4 kbSalI-BamHI fragment from pSC-M (Huber et al., 1991), the 1.7 kbHpaI-PacI fragment from pCG-F, and the 1.6 kb SmaI-XbaI fragment frompSC-H (Huber et al., 1991). pCG, a eukaryotic expression vectorcontaining a SV40 origin of replication and a CMV promoter/enhancer, wasconstructed by deletion of the L gene as well as the downstream β-globinsplice site of PSC-L (Huber et al., 1991; Severne et al., 1988) andsubsequent insertion of the β-globin splice site (from pSG5 Stratagene)upstream of a new polylinker. The pCG-based plasmid pCG-F contains aninsert consisting of the entire F gene. Filters were washed in 2×SSC at20° C. for 10 min and twice in 2×SSC, 1% SDS at 65° C. for 30 min. Bandswere visualised by autoradiography.

To analyse the expression of the MV N and P proteins by Westernblotting, cells were washed with PBS and cytoplasmic extracts wereprepared using 300 μl lysis buffer (50 mM Tris-HCl pH 8, 62.5 mM EDTA,1% NP-40, 0.4% deoxycholate, 100 μg/ml phenylmethylsulfonyl fluoride,and 1 μg/ml Aprotinin). About 1/60 of the total lysates was run onSDS-8% PAGE and blotted onto Immobilon-P membranes. As first antibodies,either the rabbit polyclonal anti-N antibody #179 (kindly provided by C.Oervell prepared according to standard procedures) in a 6000-folddilution in TBST (10 mM Tris-HCl pH 7.2-8, 150 mM NaCl, 0.05% Tween 20)or the rabbit polyclonal anti-P antibody #178 (Oervell and Norrby, 1980)in a 3000-fold dilution in TBST was used. The second antibody was aswine anti-rabbit antibody coupled to horseradish peroxidase allowingthe visualisation of the bands by the enhanced chemiluminescence kit(ECL™ Amersham Life Science, RPN 2106).

For immunofluorescence microscopy, 293-3-46 cells were seeded for arescue experiment on 24 mm×24 mm glass cover slips in 35 mm wells,cultured overnight and transfected as described above. 3 days aftertransfection, cells were permeabilized with acetone:methanol (1:1) andindirect immunofluorescence was performed essentially as described(Hancock et al., 1990; Oervell and Norrby, 1980), except that PBS wassupplemented with 1 mM MgCl₂ and 0.8 mM CaCl₂ and that p-phenylendiaminewas omitted from the mountant. Viral M and H proteins were detectedusing mouse monoclonal anti-M-16BB2 and anti-H-I29 antibodies(Sheshberadaran et al., 1983) and rabbit anti-mouse IgG [F(ab′)₂]antibodies coupled to rhodamine (Pierce, 31666).

EXAMPLE 7 Genomic and Antigenomic Plasmids Specifying Mini-, Midi-, andFull Length Replicons

The plasmid constructs used in this study are shown in FIG. 2.p107MV(−):CAT and p107MV(+): CAT specify genome- and antigenome-senseRNAs, respectively, in which all MV coding regions are preciselyreplaced by the CAT coding region. In MV-infected cells or in helpercells (see below), they give rise to mini-replicons and to capped andpolyadenylated CAT mRNA comprising the 5′N and the 3′L noncoding region.p(+)NP:CAT, containing in addition also the MV N and P coding regions intheir ordinary MV sequence context, gives rise to midi-replicons. Fulllength or partially deleted antigenomic or genomic RNAs are specified byp(+)MVΔ5F, p(+)MV and p(−)MV: For all these plasmids, transcription withT7 RNA polymerase yields RNAs bearing the authentic nucleotides of theviral genomic and antigenomic termini, respectively (Sidhu et al.,1995). Correct initiation was accomplished by direct fusion of the T7promoter (devoid of its transcribed part) to the genomic and antigenomicsequence. Starting all transcripts with the MV-specific nucleotides ACCrather than the T7-specific GGG reduces the RNA yield by about one orderof magnitude, as revealed by in vitro transcription studies usingprecursor plasmid constructs. To mediate formation of the correct MV 3′termini, the hepatitis delta virus genomic ribozyme sequence (Perrottaand Been, 1990) was cloned immediately adjacent to the MV 3′ terminalnucleotides; the introduction of T7 terminators increased the efficiencyof self-cleavage.

EXAMPLE 8 Helper Cells Stably Expressing MV N and P Protein as Well asT7 RNA Polymerase

The human embryonic kidney cell line 293 was chosen because it is highlypermissive for MV. In addition, these cells can be efficientlytransfected by the calcium phosphate coprecipitation method; 30 to 60%of the cells stained blue 24 hours after transfection with a plasmidencoding β-galactosidase.

Following cotransfection of 293 cells with pSC6-N, pSC6-P andpSC6-T7-NEO as described in the Examples, about 100 colonies wereexpanded under neomycin selection. The expression of N and P wasscreened by Western blotting, and the activity of T7 RNA polymerase wasevaluated by transfection with a reporter plasmid containing the fireflyluciferase coding region under control of a T7 promoter. Many clonesexpressed high levels of P, but only few coexpressed N efficiently. FIG.3 shows N and P expression of two selected cell lines at levelscomparable to that of MV-infected 293 cells; T7 RNA polymerase activitydetected in clone 293-3-46 was among the highest of all clones whereasit was about 100 times lower in clone 293-3-64 which turned out not torescue MV. A third cell line, 293-3-43, expressing the three proteins atlevels comparable to 293-3-46 was also active in rescue.

The expression of the introduced genes did not reduce the susceptibilityfor MV infection. The helper cell line 293-3-46 principally used MVrescue, although growing at a rate 2-3 times slower in comparison to theparent 293 line, proved to be very stable and fully functional aftermore than 80 cell splittings at dilutions 1:4 to 1:8.

EXAMPLE 9 From MV Mini-Replicon Rescue Using Helper MV to MV RescueUsing Helper Cells 293-3-46

The MV rescue system was developed stepwise, permitting to functionallytest all components. On one side, MV-dependent rescue of mini- and latersuccessively longer midi-replicons was ascertained by CAT reporterassays. Similarly, on the other side, the functionality of the 293-3-46cells was compared t the MV-based help described before (Sidhu et al.,1995).

The mini-replicon rescue test is shown schematically in FIG. 4A. Smalltranscripts from p107MV(−):CAT, p107MV(+):CAT (Sidhu et al., 1995) andlater longer transcripts, e.g. generated from p(+)NP:CAT (FIG. 2),behaved like mini- and midi-replicons, respectively. They wereencapsidated, transcribed to produce CAT, replicated and packaged intovirion particles to infect new cells. During the first 2 to 4 infectioncycles, they massively amplified whereas in later cycles replication ofboth MV and the mini-replicons was curtailed, as observed for naturallyoccurring DI RNAs (Re, 1991). Analyses of the amplified RNAs showed thatthe encapsidated replicons and the CAT transcripts contained therespective different MV-specific terminal regions (Sidhu et al., 1995).Most importantly, it turned out that for efficient function, the totalnumber of nucleotides of the replicons had to be a multiple of six, arequirement—termed the rule of six—previously found essential fornatural and slightly modified SeV DI RNAs of the copy-back type (Calainand Roux, 1993). Adherence to this rule was crucial for the constructionof plasmids specifying a variety of mini- and midi-replicons such asthose shown in FIG. 2. This was also the case for full lengths clones.

The helper function of stably transfected cell clones was tested withthe set-up represented in FIG. 4B, using however either plasmidp107MV(−):CAT, p107MV(+):CAT or p(+)NP:CAT (FIG. 2) instead of p(+)MV.As shown in FIG. 5, CAT, activity arose in the transfected cells,although at levels considerably lower than in 293 cells infected with MVand cotransfected directly with mini- or midi-replicon RNA. Thecotransfection of plasmid pEMC-La encoding the MV L protein was anabsolute requirement. As expected, low background CAT activity wasdetected when the plus-sense mini-replicon construct was used. The twoconstructs containing only the CAT reading frame in the plus- andminus-sense elicited about equal amounts of CAT activity; themidi-replicon construct gave rise to roughly 100 times less CAT activitythan the mini-replicon.

The transfection protocol was optimised in terms of maximal achievableCAT activity, using mini- and midi-replicon plasmids. Then, the fulllength constructs p(+)MV and p(−)MV were tested. About 10⁶ cellscontained in each 35 mm well were transfected and we estimate that aboutone tenth of these actually received full length as well as theL-encoding plasmids. Usually, following cotransfection of p(+)MV andpEMC-La, 1 to 6 syncytia developed after 2 to 3 days in each well. Nosyncytia were found when the latter was omitted or when the p(−)MVplasmid was used. The rescue experiments were carried out by differentexperimenters using different DNA preparations. The efficiency wasslightly viable, but at least 30% of the transfected wells revealedrescue. FIG. 6 shows typical syncytia formed in these experiments,viewed either directly (phase contrast, 6A) or after fixation of cellsgrown on cover slips (phase contrast, 6B, or immunofluorescence of thesame area, 6C).

EXAMPLE 10 Characterisation of Rescued MV

First, it had to be ascertained that the rescued MVs contained thegenetic tag which had been introduced into the MV full length plasmidclones. The 3 nt tag indicated in FIG. 2 originated from a variant 176nt N/P noncoding gene boundary region (NCGB) recovered from theSSPE-derived MV replicating in IP-3-Ca cells (Ballart et al., 1990).Rescued viruses were amplified in Vero cells, either directly from thetransfected cells or after plaque purification; the products recoveredby reverse transcription followed by polymerase chain reaction (RT-PCR)were analysed by cycle sequencing. FIG. 7 shows an example of theseanalyses, revealing the AG tag instead of CA in the Edmonston B strainpassaged in our laboratory.

We did not analyse the entire sequence of rescued MVs to exclude anyerror introduced either during the assembly of the anti-genomic plasmidclones or during T7 RNA polymerase transcription in the rescue step.However, major deleterious changes could be ruled out by analysing thereplication behaviour of the rescued virus in comparison to that of theEdmonston B strain. FIG. 8 shows that both the speed of replication aswell as the final titers reached in repeated experiments wereindistinguishable between single plaque-purified normal (MV EdB) andrescued (MV tag EdB) viruses. The apparent different at day 1 afterinfection was not a consistent observation. Non-plaque-purified virusstocks gave similar results.

EXAMPLE 11 MV Missing 504 Nucleotides in the F Gene 5′ Noncoding Region

As a first application of the reverse genetics system, we deleted 504nucleotides, thus generating a shortened genome compatible with the ruleof six mentioned above. This eliminated almost the entire F gene segmentof the long enigmatic noncoding MF NCGB which is typical for MV and theother morbilliviruses, whereas the representatives of the other twogenera of the subfamily Paramyxovirinae, paramyxovirus and rubulavirus,contain only a short NCGB. Remarkably, it was viable and moreover itreplicated in cell culture at a rate indistinguishable from that of theEdmonston B and the rescued nondeleted MV strain (FIG. 8, MVΔ5F EdB). Todetermine the size of the F gene derived RNAs, the MV-specific mRNAinduced by these plaque purified viruses was analysed, using probesspecific for the F and for the M and H genes situated up- and downstreamof F, respectively. Indeed, as shown in FIG. 9, the F mRNA as well asthe MF and FH bicistronic RNAs are consistently shorter in cellsinfected with the MVΔ5F EdB variant.

EXAMPLE 12 MVs Expressing CAT Activity

To explore the feasibility to express foreign proteins from engineeredMV we inserted a CAT reading frame flanked by intercistronic regionsinto the MV antigenomic cDNA sequence; two positions were tested, on onehand between the N and the P and on the other hand between the H and theL gene (FIG. 10, p(+)MPCATV and p(+)MHCATV, respectively). Theintercistronic region flanking the CAT reading frame was devicedaccording to the intercistronic N/P gene boundary region, but containsadditional restriction sites unique in the entire plasmid, suitable forfurther manipulations. From these constructs, recombinant MVs expressingCAT activity were rescued with about the same efficiency as from thestandard and the deleted constructs p(+)MV and p(+)MΔSFV, respectively.As expected from the natural transcription gradient typical for allMononegavirales, p(+)MHCATV expressed somewhat less CAT activity thanp(+)MPCATV. Most importantly, the CAT expression of the recombinantviruses seems to be remarkably stable as revealed from the experimentmentioned in the legend to FIG. 12 in which an overall amplification ofthe recombinant viruses of at least 10³⁰ was achieved. We actually hadexpected that viruses rescued from p(+)MPCATV would be less stable thanthose from p(+)MHCATV, because in the former the transcription of allgenes following the inserted CAT are expected to be lower than normalwhereas in the latter only the L gene transcription should be lower.Apparently, the position of the insert does not greatly affect theviability of the rescued viruses. However, no competition experimentswith standard MV have been carried out so far. Furthermore, it has to beexpected that recombinant viruses expressing proteins which activelyinterfere with MV replication will turn out to maintain the insertedgene less faithfully.

It should be mentioned here that insertion of a foreign coding sequencewithin existing MV genes should be even less harmful for the viralreplication than by creating new transcription units as in theconstructs discussed above. The general inability of the eukaryotictranslation machinery to express more than one reading frame from a mRNAcan in principle be overcome by (at least) two devices: the stop/restartmechanism and internal ribosome entry sites (IRES). Both mechanisms areactually used in special cases for natural protein expression. Anexample of the first is represented by the translation of the M2polypeptide in Influenza B virus (Horvath, C. M., Williams, M. A., andLamb, R. A. (1990) Eukaryotic coupled translation of tandem cistrons;identification of the influenza B virus BM2 polypeptide. EMBO J. 9,2639-2947). For the second mechanism, many recognized natural precedentsexist, most notably the IRES of Picornaviridae (Sonenberg, N. (1990)Poliovirus translation. Curr. Top. Microbiol. Immunol. 161, 23-47), butalso IRES in cellular mRNAs such as that specifying BiP (Sarnow, P.(1990) Translation of glucose-regulated protein 78/immunoglobulinheavy-chain binding protein mRNA is increased in poliovirus-infectedcells at a time when cap-dependent translation of cellular RNA isinhibited). All of these cited types of device have been explored in thecontext of the MV N and H genes, using as coding regions downstream ofthe MV N and H reading frames those yielding CAT and firefly luciferase,respectively, as reporters. The whole bicistronic constructs wereexpressed from conventional expression plasmids in primate cells andyields of reporter proteins ranging between 10 and 100% in comparison tothe proteins encoded by the upstream reading frames were obtained(Diploma theses, University of Zurich, composed by A. Cathomen (1991)and O. Peter (1992)).

EXAMPLE 13 MV Chimera Bearing the VSV Envelope Protein

To explore the feasibility to rescue genetically stable chimericMononegavirales in which the envelope proteins of one virus are replacedby the those of another virus p(+)MGV and pMG/FV (FIG. 10) wereconstructed. In the former construct the entire MV F and H codingregions were replaced by that encoding the VSV G protein which fulfillsa receptor binding and a fusion function analogous to those of the Mv Hand F proteins, respectively. The latter construct was deviced such thata fusion protein is created containing the large exterior part and thetransmembrane region from the VSV G protein fused to the cytoplasmictail of the MV F protein which is thought to interact specifically withthe MV M protein. Indeed, chimeric viruses could be recovered from bothconstructs which could be distinguished from each other only by slightlydifferent cytopathic effects (which are both drastically different fromthose elicited by MV) and by the fact that in cells infected by thevirus rescued from the latter construct the fusion protein could berevealed by Western blotting not only by antibodies directed to the VSVG exodomain by also to antibodies directed against the MV F cytoplasmictail. Both chimera replicated, as determined by end point dilutions, toreasonably high titers only about one order of magnitude lower than thetiters obtained by MV. In addition, they showed the biologicalspecificities expected: they readily infect rodent cells (which do notexpress a MV receptor) such as BHK (FIGS. 11, 12) where they formabundant cytoplasmic and nuclear RNPs typical for MV (FIG. 11) as wellas pleomorphic particles resembling MV virions (FIG. 12) completelydifferent from the tight shell- or cigar-like VSV virions (FIG. 13)thought to be shaped primarily by the VSV M protein.

Considering the fact that MV and VSV are only very distantly relatedMononegavirales and indeed belong to different families (Paramyxoviridaeand Rhabdoviridae, respectively), it seems quite likely that manydifferent chimera involving more closely related Mononegavirales can becreated and it appears not unrealistic that also chimera containingenvelope proteins targeting particular cell receptors can be developed.

EXAMPLE 14 MVs Expressing Green Flourescent Protein (GFP)

To demonstrate that other genes than the CAT gene can be expressed in arecombinant vector in accordance with the present invention, thesequence encoding GFP (Chalfie et al. Science 263 (1994), 802-805) wasinserted into the same position as the CAT gene in vector p(+) MPCATV,resulting in recombinant vector p(+)MPGFPV; see FIG. 10B2.

In addition, the GFP coding sequence was inserted upstream of the N genegiving rise to recombinant vector4 P(+) MGFPNV (FIG. 10B2) making surethat the rule of six was not violated and using in principle a similargene boundary like segment as for the CAT constructs. In fact, aparticularly strong expression of the GFP was achieved in this way asdetected by visual evaluation of the expressed protein. It was evenpossible to express two foreign coding sequences at the same time in onerecombinant construct as has been demonstrated with MV expressing twocopies of GFP at different positions.

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1. An isolated batch of infectious RNA viruses for use as a vaccine,said infectious RNA viruses each comprising: (a) the entire (−)-strandsequence of a non-segmented negative-strand RNA virus of the familyParamyxoviridae which obeys the rule of six; operatively linked to (b) aforeign expressible RNA fragment, wherein the length of the genome ofthe infectious RNA virus is an integral multiple of six, and furtherwherein the infectious RNA virus is obtainable by the steps of: (1)introducing into a host cell a plasmid comprising a cDNA molecule, saidcDNA molecule comprising the entire (+)-strand sequence of saidnegative-strand RNA virus operatively linked to an expression controlsequence, which allows the synthesis of antigenomic RNA transcriptsbearing the authentic 3′-termini without help of a helper virus, saidhost cell comprising (i) transfected nucleic acid molecules containingnucleic acid sequences that allow expression of an exogenous RNApolymerase, a viral N protein and a viral P protein, and (ii) a nucleicacid molecule containing nucleic acid sequence encoding a viral Lprotein; and (2) recovering and isolating the infectious RNA virus,wherein the isolated batch of infectious RNA viruses is free of helpervirus.
 2. The isolated batch of infectious RNA viruses of claim 1,wherein the RNA viruses are measles viruses.
 3. The isolated batch ofinfectious RNA viruses of claim 1, wherein the RNA viruses are mumpsvirus.
 4. The isolated batch of infectious RNA viruses of claim 1,wherein said foreign expressible RNA fragment is inserted into anon-coding region of said (−)-strand sequence and flanked by viraltranscription control sequences.
 5. The isolated batch of infectious RNAviruses of claim 1, wherein said foreign expressible RNA fragmentencodes at least one immunogenic epitope.
 6. The isolated batch ofinfectious RNA viruses of claim 1, wherein said foreign expressible RNAfragment encodes at least one immunogenic epitope of at least onepathogen or an envelope protein.
 7. The isolated batch of infectious RNAviruses of claim 1, wherein said foreign expressible RNA fragmentencodes a protein from a virus, a bacterium, or a parasite.
 8. Anisolated batch of infectious RNA viruses for use as a vaccine, saidinfectious RNA viruses each comprising: (a) the entire (−)-strandsequence of a non-segmented negative-strand RNA virus of the familyParamyxoviridae which obeys the rule of six; operatively linked to (b) aforeign expressible RNA fragment, wherein the length of the genome ofthe infectious RNA virus is an integral multiple of six, and furtherwherein the infectious RNA virus is obtainable by the steps of: (1)introducing into a host cell a plasmid comprising a cDNA molecule, saidcDNA molecule comprising the entire (+)-strand sequence of saidnegative-strand RNA virus operatively linked to an expression controlsequence, which allows the synthesis of antigenomic RNA transcriptsbearing the authentic 3′-termini without help of a helper virus, saidhost cell comprising (i) stably transfected nucleic acid moleculescontaining nucleic acid sequences that allow genomic expression of anexogenous RNA polymerase, a viral N protein and a viral P protein, and(ii) a nucleic acid molecule containing nucleic acid sequence encoding aviral L protein; and (2) recovering and isolating the infectious RNAvirus, wherein the isolated batch of infectious RNA viruses is free ofhelper virus.
 9. The isolated batch of infectious RNA viruses of claim8, wherein the RNA viruses are measles viruses.
 10. The isolated batchof infectious RNA viruses of claim 8, wherein the RNA viruses are mumpsviruses.
 11. The isolated batch of infectious RNA viruses of claim 8,wherein said foreign expressible RNA fragment is inserted into anon-coding region of said (−)-strand sequence and flanked by viraltranscription control sequences.
 12. The isolated batch of infectiousRNA virus of claim 8, wherein said foreign expressible RNA fragmentencodes at least one immunogenic epitope.
 13. The isolated batch ofinfectious RNA viruses of claim 8, wherein said foreign expressible RNAfragment encodes at least one immunogenic epitope of at least onepathogen or an envelope protein.
 14. The isolated batch of infectiousRNA viruses of claim 8, wherein said foreign expressible RNA fragmentencodes a protein from a virus, a bacterium, or a parasite.